In recent years ceramic materials have found a wide variety of applications in many industries. Extensive efforts have been directed toward the development and manufacture of ceramic parts that exhibit the desirable physical properties of the constituent materials, e.g., hardness, maintenance of structural integrity at high temperatures, and chemical inertness. Efforts have also been directed toward the elimination of defects which often result in failure of ceramic parts. These potential failures represent a significant impediment to the increased use of ceramic materials in certain applications, and can often be attributed to small cracks or voids resulting from incomplete packing of the precursor powders. One solution to this problem s the manufacture of fine, monodispersed powders which can be packed tightly, thereby reducing the void spaces between particles. It has been suggested, by E. A. Barringer and H. K. Bowen in "Formation, Packing and Sintering of Monodispersed TiO.sub.2 Powders," J. Amer. Ceram. Soc. 65, C-199 (1982), that an `ideal` ceramic powder for producing a high quality part would be of high purity and contain particles which are monodispersed, spherical, nonagglomerated and of a particle size ranging from about 0.1 to about 1.0 micron in diameter.
Using fine ceramic powders in engineered ceramic parts offers a number of advantages. For example, as a ceramic powder is densified, adjacent particles generally fuse into grains. In general, the grain size is governed by the crystallite size within the particles from which the part is prepared, that is to say, the grain size is generally larger than the size of the crystallites from which a part is fabricated. Thus, the densification of fine particles composed of fine crystallites presents the opportunity to produce fine-grained bodies. An additional advantage in the use of ceramic powders with a fine uniform crystallite size is that the temperatures required to densify the powders are often reduced. On an industrial scale, this can result in a considerable savings in energy.
The relationship between grain size and physical integrity has also been investigated. For example, A. D. Osipov et al. researched this relationship for boron carbide bodies in "Effect of Porosity and Grain Size on the Mechanical Properties of Hot-Pressed Boron Carbide," Sov. Powder Metall. Met. Ceram. (Engl. Transl.) 21(1), 55-8 (1982). The authors found that parts exhibiting a finer grain size were significantly stronger than parts consisting of coarse grains. Thus, boron-based systems can clearly profit from control of grain size.
In view of these findings considerable research has been devoted to developing methods and means of producing uniform, fine-sized ceramic powders. Commercial production of ceramic powders has typically been achieved batchwise, through attrition milling, acid leaching, and size classification of ceramic powders. These powders have generally been synthesized via reactions employing slow, non-uniform heating over extended time periods. For example, commercial production of boron carbide is most commonly carried out by the reduction of boric oxide with carbon in a batch electric arc furnace, as described by A. Lipp in "Boron Carbide: Production, Properties, Application," Technische Rundschau, No. 14, 28, 33 (1965) and 7 (1966). Reaction and cooldown take place over an extended period of time, on the order of days, because of the slow rate of heat conduction which controls the process. The non-uniform process conditions result in non-uniform chemical compositions and crystal sizes within the product. The sintered mass of product which results from this process requires physical size reduction in order to achieve a particle size fine enough for densification. Because of the extreme hardness of boron carbide, this size reduction step is extraordinarily difficult and expensive and results in contamination of the product with impurities picked up during milling. Acid leaching of metal impurities is necessary and further complicates the process.
Because of the problems encountered due to the slow, non-uniform heating and subsequent processing complications, researchers have sought methods of producing suitable powders directly, such that size reduction and other additional steps can be avoided. One effective method involves the direct synthesis of powders from laser-heated gases. For example, R. A. Marra and J. S. Haggerty, in their article, "Synthesis and Characteristics of Ceramic Powders Made from Laser-Heated Gases," Sci. Proc. 3, 31 (1982), describe the preparation of silicon, silicon carbide and silicon nitride powder by driving exothermic reactions involving SiH.sub.4. The result is equiaxed, monodispersed powders with particle sizes in the range of 0.01-0.1 micron. Marra and Haggerty further state that this laser-heated process can be used to produce both oxide and nonoxide ceramics such as TiB.sub.2, AlN, B.sub.4 C, and so forth.
Powders have also been synthesized from radio frequency plasma-heated gases. See, e.g., Steiger U.S. Pat. No. 4,266,977. That patent describes a gas phase pyrolysis process for manufacturing submicron sized, carbon-containing titanium diboride powders whereby titanium halide and gaseous boron source (e.g., boron trichloride) reactants are mixed with a hot stream of hydrogen produced by heating hydrogen in a plasma heater.
In another gas phase type synthesis process, Latham, Jr., in U.S. Pat. No. 3,346,338 discloses the continuous production of finely divided silicon or titanium carbide by passing a vapor of each reactant into one end of a furnace reaction zone and then recovering from the other end of the reaction zone a finely-divided carbide product.
In general, the laser- or plasma-heating of reactant gases is characterized by almost instantaneous heating rates of reactants, short reaction times (fractions of a second) with minimal exposure to high temperature, and almost instantaneous product cooling rates. The net result of the nearly instantaneous and uniform heating rates is submicron, uniformly sized ceramic particles. However, while gas phase synthesized powders possess many of the desirable qualities, they are relatively expensive to produce because of the inherently slow generation rate and high cost of equipment and gaseous raw material (e.g., boron trichloride) which they require. Thus, the gas phase routes, while academically intriguing, may not be practical for commercial use.
Another method for directly manufacturing fine ceramic powders is via the reduction of a metal oxide with a metal, the so-called "thermite reaction." For example, U.S. Pat. No. 2,834,651 discloses a batch method of producing boron carbide of fine particle size by heating a mixture of boric oxide, carbon, and magnesium. Typically, reactants are intimately mixed, loaded into a container, and the reaction initiated either by heating the entire reaction mixture to a sufficiently high temperature or through the use of fuses and the like. The thermite reaction is highly exothermic and self-propagating. Although typically fine in size, particles produced by the thermite process are of a fairly wide distribution (0.2 to 10 microns) due to non-uniform heating rates, temperatures, and reaction times at temperature. Since excess metal typically is used in these reactions, a post-treatment acid leach/wash step to solubilize and wash out residual metals is required. The ceramic powders produced by the thermite reaction are unsatisfactory for high purity applications because the powders are contaminated with residual metals. Even after repeated digestion with hot mineral acids, these are difficult to remove.
Efforts to directly produce uniform, fine powders by less expensive, more commercially practicable means have included various furnace modifications. In general these involve passing solid reactants through a heated, relatively restricted space, containing inert or reaction-compatible gases, at a variable rate according to the desired reaction and the necessity to avoid decomposition of the desired product. For example, in Serpek U.S. Pat. No. 1,212,119 discloses a vertical furnace in which a mixture of carbon and an aluminous material is heated, while either free-falling in a nitrogen atmosphere or being swept in a nitrogen stream, sufficiently to produce aluminum nitride. Another patent to Serpek, U.S. Pat. No. 1,217,842, discloses a furnace in which the gaseous current does not sweep through the reaction zone along the same path as the reactant material, but rather passes through porous walls into the reaction zone. This inhibits deposition of either reactant materials or product on the porous walls of the reactor.
Two types of vertical, "fluid wall," tubular reactors are described in a number of patents to Matovich (U.S. Pat. No(s). 3,933,434; 4,042,334; 4,044,117; 4,056,602: 4,057,396: 4,095,974: 4,199,545; and 4,234,543). These reactors have an inlet end, a reaction chamber, and an outlet end. The reaction chamber is defined as the interior of the envelope of inert fluid which protects the inside tube wall from reactants and products of reaction. The two types of reactor arise from the method in which the "fluid wall" annular envelope is generated. In one embodiment the reactor has a porous wall through which inert fluid flows radially inward of the inner surface of the reactor tube. In the other embodiment a laminar diffuser is located adjacent to the inlet end and causes a fluid directed under pressure to flow in substantially laminar fashion through the reaction chamber. This provides a protective blanket for the interior surface of the reactor tube. In general these reactors are described as being useful for a variety of chemical processes involving pyrolysis, thermolysis, dissociation, decomposition and combustion reactions of both organic and inorganic compounds.
Enomoto et al., in U.S. Pat. No. 4,292,276, discloses an apparatus for producing silicon carbide consisting mainly of beta-type crystals. It uses a vertical-type reaction vessel having an inlet for a starting material, a preheating zone, a cooling zone, and a closable outlet for a product in this order. The closable outlet allows extended reaction times, on the order of hours, for the gravity-fed briquettes, which are typically 3 to 18 mm in diameter. This design uses electrically indirect heating.
No special provisions are made with any of these reactor/furnace designs for the continuous entry of meltable solids into the reaction chamber, or for the continuous discharge of condensing fluids through the outlet end. A particular problem is encountered when using feedstocks comprising boric oxide, boric acid, or boric oxide with surface moisture, which behaves as boric acid, to produce boron-containing products Boric oxide is of particular commercial significance as a starting material for a number of these boron-containing ceramic compounds because of its relatively low cost and easy availability. The problem, however, is that boric oxide softens at about 325.degree. C., melts at about 450.degree. C., and volatilizes at above about 1400.degree. C. Boric acid goes through a melt phase at about 150.degree. C. to 175.degree. C., forming the liquid meta borate BO.multidot.OH. When the furnace designs described above are used with boric oxide, the particles go through a heating cycle from below about 150.degree. C. to above about 1400.degree. C. as they enter the furnace reaction zone, and thus are inevitably in the liquid stage at a certain place near the inlet of the reaction zone. This means that liquid boric oxide will tend to deposit somewhere near the entrance to the furnace reaction zone, which often causes plugging problems.
Even when entrainment gas (inert or reaction-compatible gas) is used to entrain fine reactant powder containing boric oxide into the reactor's reaction zone, counter-flowing thermal eddy currents within the reactor inevitably force a substantial quantity of fine reactant powder against cooler inlet surfaces, resulting in plugging due to the formation of larger agglomerates containing boric oxide. These larger agglomerates may then fall or be swept through the reaction zone to yield product agglomerates having incompletely converted inner cores of reactant.
A problem encountered specifically with the use of a "fluid wall" reactor is that of limited residence time within the reaction zone. A significant quantity of fluid is necessary to generate the annular envelope of gas which protects the reactor wall. The residence time of reactant powder transported through the reactor is highly dependent on the flow rate of gas within the reactor tube. Hence, it is expected that in carrying out a reaction between solids (such as boric oxide and carbon to synthesize boron carbide and carbon monoxide) it will be necessary to minimize the flow of unnecessary inert fluids in order to maximize reactor capacity. This is especially true if the inert fluid is expensive, such as are argon or helium.
Another problem with using the known furnace configurations is that of preventing the condensation of excess vaporized reactant (e.g., boric oxide) along the inside walls of the cooling zone in those designs having such a specified area. Excess boric oxide is typically employed in the reactant mixture because any unreacted boric oxide is soluble in water and can usually be easily washed from the product powder. When the furnace designs described above are used with excess-boric oxide-containing feeds, the exiting product contains vaporized boric oxide which goes through a cooling cycle from above 1400.degree. C. to below 325.degree. C. as it passes within the cooling zone, and thus inevitably is in the liquid stage at a certain place near the inlet of the reactor cooling zone. This means that liquid boric oxide will tend to deposit and solidify within the inlet of the cooling zone, again often causing plugging problems and preventing continuous operation.
Some other problems encountered are related to the final product. First, it is extremely difficult to produce metastable products, such as the boron-rich boron carbides, with many of the known furnaces because the cooling, called "quenching," is not rapid enough to essentially stop the reaction at a metastable point. Finally, the known furnaces may not be capable of producing the desired uniformity and submicron size for optimal performance of a densified ceramic piece. Unlike the laser method, which offers extremely large, almost instantaneous and uniform temperature differentials, the known furnaces offer environments in which the temperature gradients are much more gradual and significantly less uniform, and thus there is opportunity for crystallite growth and therefore an increase in grain size in the densified piece.
Thus, it would be desirable to develop an apparatus and method of producing uniform, fine ceramic powders, of preferably submicron diameters and high purity. Such an apparatus and method should preferably be adaptable to the use of boric oxide as a feedstock and eliminate or reduce the problems of deposition on the furnace walls, at any point in the process, of either feedstocks or products. It should also preferably be adaptable to the production of metastable-form boron-containing ceramic powders and boron-containing composite ceramic powders.